In many industrial product treatment processes, safe disposal of byproducts of the processes is a significant problem. For example, in material finishing operations in which paint, enamel, lacquer, etc. are sprayed onto the product, the air ambient the products must be cleansed of the overspray before the air is exhausted into the surrounding atmosphere. The air may be cleansed of the overspray by means of water wash systems such as that shown in U.S. Pat. No. 4,484,513, having the same assignee as the present invention. The water-laden sludge formed of the overspray paint particles is initially separated from the water in the wash by means of skimming devices such as that shown in U.S. Pat. No. 4,432,870, also having the same assignee as the present application. This patent discloses the use of a separate tank to which sludge-laden water is conducted for removal of sludge and aeration of the water before the water is returned to the reservoir for recycling in the water wash or is otherwise disposed of. In the device of the patent, the sludge is urged to the surface of the tank through water circulation means, and is then skimmed off the surface of the water into a collection bin for recycling, disposal, etc.
In some instances rather than skimming the paint sludge from the water, the water and sludge are conveyed to a vacuum-type filter with the water and sludge being deposited on the top of a screen with a vacuum being applied under the screen to pull the water through the screen. The sludge is left on top of the screen. This system has a tendency to have the sludge clog the screens and is a high maintenance system. It will be appreciated that a very high volume of water to sludge is being conveyed to the screens and this water and the vacuum tend to cause the sludge to enter into and clog the screen holes. Another continuous process in which a high volume flow of water from the spray tank reservoir is used to convey the sludge particles is a centrifuge system in which the liquid and sludge particles are introduced into the top of the centerfuge and spun downwardly with the water being thrown off and with the sludge exiting through a bottom orifice. The sludge has a tendency to clog the centerfuge orifice and this also results in high maintenance problems. It will be appreciated that the sludge is a very cohesive and sticky material and may clog such orifices.
Batch type of processes have been employed to dewater sludge. For instance, sludge is shoveled from the bottom of a spray tank reservoir into drums which are transported into a filter press having press screens which squeeze the sludge. The screens must be cleaned very regularly or they become clogged.
There is a need for an improved method and apparatus for handling and dewatering sludge preferably on a continuous basis rather than a batch basis and, which can operate with reduced maintenance, as contrasted to the above-described systems. Preferably, such a system will remove sufficient water from the sludge that it may be classified as "dried sludge" rather than "liquid sludge." The changing of the sludge classification from "wet" to "dried" sludge greatly affects the methods by which it can safely be handled and stored. Specifically, a distinction is drawn between "dried" sludge and "liquid" sludge. Dried sludge is sludge that has been dewatered so that it can be transported and handled as a solid material, while liquid sludge is readily pumpable and must be transported by a closed vessel which will not allow any leakage or spillage. Dewatered sludge may be transported in open dump trucks. Further, neither containers holding free liquids nor noncontainerized liquid sludge may be placed in a landfill unless the landfill has an impermeable liner and leachate collection means. Consequently, greater safety and convenience can be obtained if the sludge is dewatered before disposal. Tests for classification of sludge are available from governmental agencies such as Environmental Protection Agencies.
Accordingly, it is a principal object of the present invention to provide an improved method and apparatus for dewatering sludge so that it can be safely and conveniently handled and stored.
It is another object to provide a method and apparatus which will dewater sludge sufficiently that it will be classified as dried sludge in an economic and convenient manner.
A still further object is to provide a method and apparatus that provides for the collection of both the solid sludge and liquid dewatered therefrom.
These objects and others will become apparent from the following description of the invention in which the water sludge suspension in the spray tank reservoir is conveyed to a skimming apparatus which separates the floating sludge from the water suspending the same. The reservoir water having been cleaned of the sludge is returned to the reservoir for reuse. The skimmed sludge is still too wet to be classified as "dried sludge" under most govermental standards so that it needs to be further dewatered before it can be classified as "dried sludge"; and to this end, the skimmed sludge is conveyed by conveying means to a sludge dewatering means which removes sufficient water from the skim sludge to have it classified as "dried sludge." The preferred dewatering means oscillates the sludge or produces a squeezing action on the sludge on a screen to exude the occluded water. The preferred screen is inclined and oscillated or vibrated to work the sludge downwardly while exuding water therefrom.